Method for reducing contact resistance in semiconductor structures

ABSTRACT

Semiconductor structures and methods reduce contact resistance, while retaining cost effectiveness for integration into the process flow by introducing a heavily-doped contact layer disposed between two adjacent layers. The heavily-doped contact layer may be formed through a solid-phase epitaxial regrowth method. The contact resistance may be tuned by adjusting dopant concentration and contact area configuration of the heavily-doped epitaxial contact layer.

BACKGROUND

Many developments in both semiconductor structures and manufacturingprocesses have contributed to reducing the size and increasing theperformance of integrated circuits. One recent advance in semiconductorstructures has been the introduction of a transistor structure referredto as a finFET. FinFET transistors typically have advantages such asgreater channel control, reduced short channel effect, and lowersubthreshold leakage currents.

Integrated circuits often include electrical components in addition totransistors, such as, for example, diodes, capacitors, and resistors,each of which may be combined with FinFETs to form an electricalcircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the common practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is an isometric view of an exemplary semiconductor devicestructure.

FIG. 1B shows a top view of an exemplary transistor region.

FIGS. 2A-2E show cross-sectional views of a partially fabricated finFETafter each of a series processing operations according to thisdisclosure.

FIGS. 3A-3B show cross-sectional views of a partially fabricated finFETafter each of a series of processing operations for forming aheavily-doped crystalline layer over source/drain (S/D) regionsaccording to this disclosure.

FIGS. 4A-4B show cross-sectional views of a partially fabricated finFETafter each of a series of processing operations for forming aheavily-doped crystalline layer over S/D regions and fin sidewalls of anexample transistor region, according to this disclosure.

FIG. 5 is a flow diagram illustrating an exemplary method, according tothis disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over a second feature in the description that followsmay include embodiments in which the first and second features areformed in direct contact, and may also include embodiments in whichadditional features may be formed that are between the first and secondfeatures, such that the first and second features are not in directcontact. In addition, the present disclosure may repeat referencenumerals and/or letters in the various examples. This repetition doesnot in itself dictate a relationship between the various embodimentsand/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The acronym “FET,” as used herein, refers to a field effect transistor.A very common type of FET is referred to as a metal oxide semiconductorfield effect transistor (MOSFET). Historically, MOSFETs have been planarstructures built in and on the planar surface of a substrate such as asemiconductor wafer. But recent advances in semiconductor manufacturinghave resulted in the use vertical structures.

The term “finFET” refers to a FET that is formed over a fin that isvertically oriented with respect to the planar surface of a wafer.

“S/D” refers to the source and/or drain junctions that form two of thefour terminals of a FET.

The expression “epitaxial layer” herein refers to a layer or structureof single crystal material. Likewise, the expression “epitaxially grown”herein refers to a layer or structure of single crystal material.Epitaxially-grown material may be doped or undoped.

The term “nominal” as used herein refers to a desired, or target, valueof a characteristic or parameter for a component or a process operation,set during the design phase of a product or a process, together with arange of values above and/or below the desired value. The range ofvalues is typically due to slight variations in manufacturing processesor tolerances.

The term “vertical,” as used herein, means nominally perpendicular tothe surface of a substrate.

Various embodiments in accordance with this disclosure provide reducedcontact resistance compared with conventional processes andsemiconductor structures. Specifically, contact resistance may bereduced by incorporating a low contact resistance layer betweenepitaxial material and silicide contact material in semiconductordevices, such as field-effect transistors (FETs), horizontalgate-all-around (HGAA) structures, and channel-on-oxide (COO)structures. In a fin field-effect transistor (finFET) structure, forexample, a heavily-doped low contact resistance layer may be interposedbetween the epitaxial portions and silicide contact portions of the S/Dregions. A lower contact resistance may provide for increased powerdensity which in turn provides for improved transistor performance. Bysuitably adjusting growth and doping parameters, very high doping levelscan be achieved in the low contact resistance layer with minimaldiffusion into the underlying semiconductor structure.

Before describing the embodiments related to the design of finFET S/Dregions, an example fabrication process for a finFET is presented. FIGS.1A-2E provide various views of a semiconductor device that includesfinFETs during various stages of fabrication. The fabrication processprovided here is exemplary, and many other steps may be performed thatare not shown in these figures.

Illustrated in FIG. 1A is an isometric view of a semiconductor structure100. Semiconductor structure 100 includes finFETs. Semiconductorstructure 100 includes a substrate 102, a plurality of fins 104, aplurality of isolation structures 106, and a gate structure 108 that isdisposed over the sidewalls and top surface of each of fins 104. Fins104 and isolation structures 106 have top surfaces 114 and 118,respectively. Gate structure 108 includes a gate dielectric structure115, and a gate electrode structure 117. In alternative embodiments, oneor more additional layers or structures may be included in gatestructure 108. FIG. 1A shows a hard mask 120 disposed on a top surfaceof gate electrode layer 117. Hard mask 120 is used to pattern, such asby etching, gate structure 108. In some embodiments, hard mask 120 ismade of a dielectric material, such as silicon nitride. The isometricview of FIG. 1A is taken after the patterning process (e.g., etching) ofa gate dielectric layer and a gate electrode layer to form gatestructure 108. FIG. 1A shows only one gate structure 108. Those skilledin the art will understand that typical integrated circuits contain aplurality of such, and similar, gate structure(s).

Each of the plurality of fins 104 shown in FIG. 1A includes a pair ofS/D terminals. For ease of description, a first one of the pair of S/Dterminals is referred to as a source region 110 _(S) and a second one ofthe pair of S/D terminals is referred to as a drain region 110 _(D),where S/D terminals are formed in, on, and/or surrounding fin 104. Achannel region 112 of fin 104 underlies gate structure 108. Gatestructure 108 has a gate length L, and a gate width (2×H_(F)+W), asshown in FIG. 1A. In some embodiments, the gate length L is in a rangefrom about 10 nm to about 30 nm. In some other embodiments, the gatelength L is in a range from about 3 nm to about 10 nm. In someembodiments, the fin width W is in a range from about 6 nm to about 12nm. In some other embodiments, the fin width W is in a range from about4 nm to about 6 nm. Gate height H_(G) of gate structure 108, measuredfrom fin top surface 114 to the top of gate structure 108, is in a rangefrom about 50 nm to about 80 nm, in some embodiments. Fin height H_(F)of fin 104, measured from the isolation structure top surface 118 to fintop surface 114, is in a range from about 25 nm to about 35 nm, in someembodiments.

Substrate 102 may be a silicon substrate. Alternatively, substrate 102may comprise another elementary semiconductor, such as germanium; acompound semiconductor including silicon carbide, gallium arsenide,gallium phosphide, indium phosphide, indium arsenide, and/or indiumantimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs,AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In anembodiment, substrate 102 is a semiconductor on insulator (SOI). Inanother embodiment, substrate 102 may be an epitaxial material.

Isolation structures 106 are made of a dielectric material and may beformed of silicon oxide, spin-on-glass, silicon nitride, siliconoxynitride, fluorine-doped silicate glass (FSG), a low-k dielectricmaterial, and/or other suitable insulating material. Isolationstructures 106 may be shallow trench isolation (STI) structures. In anembodiment, the isolation structures are STI structures and are formedby etching trenches in substrate 102. The trenches may then be filledwith insulating material, followed by a chemical mechanical polish (CMP)and etch-back. Other fabrication techniques for isolation structures 106and/or fin 104 are possible. Isolation structures 106 may include amulti-layer structure, for example, having one or more liner layers.

Fins 104 are active regions where one or more transistors are formed.Fin 104 may comprise silicon or another elementary semiconductor, suchas germanium; a compound semiconductor including silicon carbide,gallium arsenide, gallium phosphide, indium phosphide, indium arsenide,and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP,AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof.Fins 104 may be fabricated using suitable processes includingphotolithography and etch processes. The photolithography process mayinclude forming a photoresist layer (resist) overlying the substrate(e.g., on a silicon layer), exposing the resist to a pattern, performingpost-exposure bake processes, and developing the resist to form amasking element including the resist. The masking element may then beused to protect regions of the substrate while an etch process formsrecesses into isolation structures 106, leaving protruding fins. Therecesses may be etched using reactive ion etch (RIE) and/or othersuitable processes. Numerous other methods to form fins 104 on substrate102 may be suitable. Fins 104 may comprise epitaxial material, inaccordance with some embodiments.

Gate structure 108 may include a gate dielectric layer 115, a gateelectrode layer 117, a spacer layer 111, and/or one or more additionallayers. For ease of description, spacer layer 111 is not shown in FIG.1A. In an embodiment, gate structure 108 uses polysilicon as gateelectrode layer 117. Also shown in FIG. 1A is a hard mask 120 disposedon a top surface of gate electrode layer 117. Hard mask 120 is used topattern, such as by etching, gate structure 108. In some embodiments,hard mask 120 is made of a dielectric material, such as silicon nitride.

Although the isometric view of FIG. 1A shows gate structure 108 usingpolysilicon as the gate electrode layer 117, those skilled in the artwill understand that gate structure 108 may be a sacrificial gatestructure such as formed in a replacement gate process used to form ametal gate structure. The replacement gate process and many other stepsmay be performed and are not shown in these figures. The metal gatestructure may include barrier layer(s), gate dielectric layer(s), workfunction layer(s), fill metal layer(s) and/or other suitable materialsfor a metal gate structure. In other embodiments, the metal gatestructure may further include capping layers, etch stop layers, and/orother suitable materials.

Exemplary p-type work function metals that may be included in the metalgate structure include TiN, TaN, Ru, Mo, Al, WN, ZrSi₂, MoSi₂, TaSi₂,NiSi₂, WN, other suitable p-type work function materials, orcombinations thereof. Exemplary n-type work function metals that may beincluded in the metal gate structure include Ti, Ag, TaAl, TaAlC, TiAlN,TaC, TaCN, TaSiN, Mn, Zr, other suitable n-type work function materials,or combinations thereof. A work function is associated with the materialcomposition of the work function layer, and thus, the material of thefirst work function layer is chosen to tune its work function so that adesired threshold voltage V_(t) is achieved in the device that is to beformed in the respective region. The work function layer(s) may bedeposited by CVD, plasma-enhanced vapor deposition (PECVD), ALD, and/orother suitable process. The fill metal layer may include Al, W, or Cuand/or other suitable materials. The fill metal may be formed by CVD,PVD, plating, and/or other suitable processes. The fill metal may bedeposited over the work function metal layer(s), thereby filling in theremaining portion of the trenches or openings formed by the removal ofthe sacrificial gate structure.

Semiconductor device structure 100 described above includes fins 104 andgate structure 108. The semiconductor device structure 100 needsadditional processing to form various features, such aslightly-doped-drain (LDD) regions and doped S/D structures, of thetransistor utilizing structure 100. LDD regions are formed in fins 104by doping, and the term LDD regions is used to describe lightly-dopedregions disposed between the channel region of a transistor and leastone of the transistor's S/D regions. Ion implantation has been used as adoping process for many technology nodes. Embodiments in accordance withthe present disclosure are not limited to ion implantation as the dopingprocess for LDD regions.

FIG. 1B shows a top view of a transistor region 150 formed with one ofthe fins 104 of FIG. 1A and taken on a surface level with the topsurface 118 of isolation structure 106. Transistor region 150 includesS/D regions 110 _(S) and 110 _(D). Transistor region 150 also includes achannel region 112, which is part of fin 104 and is surrounded by gatestructure 108 on three sides, as shown in FIG. 1A. The channel region112 underlies the gate structure 108 and has a width (fin width) W.Depending on fabrication processing conditions and device designs, thelength of channel region 112 may be slightly different from gate lengthL. Solely for the ease of description, the length of channel region 112is denoted as gate length L. Transistor region 150 also includes gatedielectric layer 115 and gate electrode layer 117. FIG. 1B also showsspacers 111 formed on gate structures 108. LDD regions 113 are formed inthe top surface and side walls of fin 104. LDD region 113 that is shownin FIG. 1B has a width W and a length L_(S). FIG. 1B also shows anothergate structure 108 by dotted lines. This other gate structure 108 hasbeen described above as being similar and parallel to the gate structure108 and is not shown in FIG. 1A.

Referring to FIGS. 2A through 2E, various perspective andcross-sectional views of a finFET at various stages of fabricationaccording to various illustrative embodiments of the present disclosureare shown.

FIG. 2A shows two neighboring gate structures 108 formed over fin 104,taken along the cut 131 shown in FIG. 1A. Each gate structure 108includes a gate electrode 117 and a gate dielectric 115. A hard mask 120is disposed over gate electrodes 117. In some embodiments, hard mask 120is used to define the patterning of gate electrodes 117. Hard mask 120comprises any suitable material, including but not limited to, siliconnitride, SiON, SiC, SiOC, spin-on glass (SOG), a low-k film, or asilicon oxide. Such silicon oxide may be formed by any suitable methodincluding, but not limited to CVD with tetraethoxysilane (TEOS) as asource gas, plasma enhanced CVD oxide (PE-oxide),high-aspect-ratio-process (HARP) formed oxide. Channel regions 112,which are directly under the gate structures 108 are also noted in FIG.2A. A dotted line 118 indicates the level of surfaces of isolationregions 106.

FIG. 2B shows an offset spacer 116 used to expose a portion of thechannel region—i.e., LDD regions 113—to LDD ion implantation whileblocking the ion implantation from a portion of the channel regionimmediately next to the sidewalls of the gate electrode structures 117.Offset spacers 116 are formed using an etch-back technique. For example,to form offset spacer 116, a blanket offset spacer layer is firstdeposited over the substrate, including gate structures 108 which have ahard mask layer 120 over the structures. An etch-back process is thenused to remove portions of the blanket offset spacer layer to expose aportion of the channel region for ion implantation. The remainingblanket offset spacer layer forms offset spacers 116 at least on thesidewalls of gate electrode structures 117 and hardmask layer 120.Offset spacer 116 is made of a dielectric material, such as siliconoxide, SiON, or silicon nitride (SiN). In some embodiments, thedeposition process is a plasma-enhanced chemical vapor deposition(PECVD) process. Other applicable deposition processes may also be used.In some embodiments, the thickness of offset spacer 116 is in a rangefrom about 2 nm to about 4 nm. Offset spacer 116 provides an offsetdistance, which is the thickness of offset spacer 116, from channelregion 112 and prevents the dopants from being implanted in the channelregion 112.

LDD regions 113 are then formed in the fin structure between adjacentoffset spacers 116 using any suitable processes. For example, an ionimplant process is performed to form LDD regions 113, and may utilizeany suitable doping species. Although LDD regions 113 are shown as onlybeing close to the top surface of fin 104, LDD regions 113 may actuallybe close to both the top surface and sidewalls of fin 104. The LDDimplantation may be performed vertically, or tilted toward the sidewallsof fin 104. Depending on the implantation process, LDD regions 113 mayextend to a certain depth below the surfaces of fin 104. For example,LDD region 113 may extend to a depth of H_(L) below the top surface offin 104, as shown in FIG. 2B. It will be understood by those skilled inthe art that the LDD region may also extend from the sidewall surfacesof fin 104 into the interior of fin 104. Substrate 102 could have bothp-type and n-type devices. Additional processes, such as lithographypatterning processes, would be involved to protect the p-type deviceregions from dopant ions for n-type devices. The processing sequenceinvolved in forming and doping the p-type devices are well known tothose of ordinary skill in the art and are not further described in thisdisclosure.

After the dopant ions are implanted, a thermal anneal is performed todrive in and to activate the dopants. The thermal anneal may utilizerapid thermal processing (RTP) anneal, spike anneal, millisecond anneal,or laser anneal. Spike anneal operates at peak anneal temperature for atime period on the order of seconds. Millisecond anneal operates at peakanneal temperature for a time period on the order of milliseconds, andlaser anneal operates at peak anneal temperature for a time period onthe order of nanoseconds to microseconds.

FIG. 2C shows main spacers 125 formed over transistor region 150, takenalong the cut 131 shown in FIG. 1A. Main spacers 125 cover offsetspacers 116, and may also cover a top surface of gate structure 108 (notshown in FIG. 2C). The thickness of main spacer 125 is in a range fromabout 5 nm to about 10 nm, which is sufficient to protect gate structure108 and offset spacers 116 during subsequent etching of fin 104. Mainspacers 125 are formed using an etch-back technique. For example, toform main spacer 125, a blanket main spacer layer is first depositedover the substrate, including gate structures 108 which have a hard masklayer 120 over the structures. An etch-back process is then used toremove portions of the blanket main spacer layer to form an opening andexpose a portion of LDD region 113 for the subsequent fin etchingprocess. The remaining blanket main spacer layer forms main spacers 125.Main spacer 125 is made of a dielectric material, such as SiON, siliconnitride (SiN), or carbon-doped silicon nitride (SiCN). SiCN hasrelatively low etch rate against etchants, such as H₃PO₄ and HF, incomparison to SiN or SiON. In some embodiments, the deposition processis PECVD. Other applicable deposition processes may also be used. Insome embodiments, each offset spacer 116 has a width in a range fromabout 5 nm to about 10 nm. A material removal process can be performedto remove main spacer 125 that has been formed over hard mask layer 120and also over other portions of surfaces on substrate 102, for example,RIE processes and/or other suitable processes. Taken together, offsetspacers 116 and main spacers 125, are referred to as spacers 111.

FIG. 2D shows recess 127 formed in the fin between neighboring gatestructures 108, taken along the cut 131 shown in FIG. 1A. Exposedportion of fin 104 is etched using RIE processes and/or other suitableprocesses. An illustrative fin etching process may be performed under apressure of about 1 mTorr to about 1000 mTorr, a power of about 50 W toabout 1000 W, a bias voltage of about 20 V to about 500 V, at atemperature of about 40° C. to about 60° C., and using HBr and/or Cl₂ asetch gases. Also, the bias voltage used in the illustrative etchingprocess may be tuned to allow better control of an etching direction toachieve desired profiles for recess 127. In some embodiments, recess 127may be formed to have either an angular or rounded shape at its bottom.Recess 127 has bottom surface 127 t. As shown in FIG. 2D, bottom surface127 t is above the flat top surfaces 118 of isolation structure 106. Inanother embodiment, bottom surface 127 t is below the flat top surfaces118 of isolation structures 106. Spacers 111 and hard mask 120 are usedas hard masks such that recess 127 is self-aligned with the openingformed by opposing spacers 111. Height H_(R) measured from bottomsurface 127 t to isolation structure top surface 118 may be adjusted bythe manufacturer. Recesses 127 may be formed to have either an angularor rounded shape at their bottoms.

FIG. 2E shows that after recess 127 is formed, an epitaxial material isgrown in recess 127 to form epitaxial doped S/D regions, 110 _(D)′ and110 _(S)′ respectively. For ease of description, a first one of the pairof doped epitaxial S/D terminals is referred to as a source region 110_(S)′ and a second one of the pair of doped S/D terminals is referred toas a drain region 110 _(D)′. In some embodiments the dopants in dopedS/D regions 110 _(D)′ and 110 _(S)′, diffuse into LDD regions 113 duringannealing. FIG. 2E shows that epitaxial material is grown in recess 127to form doped drain regions 110 _(D)′, and for ease of description,doped source region 110 _(S)′ is not shown in FIG. 2E. At least aportion of each doped S/D region 110 _(D)′ and 110 _(S)′ is formed inrecesses 127, and therefore is also self-aligned with the openingdefined by opposing spacers 111.

In some embodiments, the epitaxial material filling recesses 127 to formdoped S/D regions, 110 _(D)′ and 110 _(S)′, is a silicon-based material.In some embodiments, the epitaxially-grown silicon-based material isformed by an epitaxial deposition/partial etch process, which repeatsthe epitaxial deposition/partial etch process at least once. Suchrepeated deposition/partial etch process is also called a cyclicdeposition-deposition-etch (CDDE) process. The deposition process formsa thin epitaxial layer of silicon-based material in recess 127 and anamorphous silicon-based material on non-crystalline surfaces. An etching(or partial etching) process removes the amorphous silicon-basedmaterial and also a portion of the silicon-based material in recesses127. As a result of the process, silicon-based material is deposited ineach of recesses 127 to form epitaxial S/D regions 110 _(D)′ and 110_(S)′, respectively.

Still referring to formation of doped S/D regions, 110 _(D)′ and 110_(S)′, in-situ doping processes may also be incorporated during or afterthe deposition of silicon-based material. For example, an n-type dopingprecursor, e.g., phosphine (PH₃) and/or other n-type doping precursors,can be used during the formation of the S/D regions of an n-typetransistor. By using the in-situ doping process, the dopantconcentration of silicon-based material can be desirably controlled. Insome embodiments, silicon-based material can be an n-type doped siliconlayer that is doped with phosphorus (Si:P). In some embodiments,silicon-based material can be an n-type doped silicon layer that isdoped with both phosphorus and carbon (Si:CP). Carbon could impede theout-diffusion of phosphorus from silicon-based material. In someembodiments, silicon-based material can be an n-type doped silicon layerthat is doped with arsenic. Other types of dopants may also be included.In some embodiments, the phosphorus dopant concentration is in a rangefrom about 7×10²⁰ atoms/cm³ to about 3×10²¹ atoms/cm³. In someembodiments, the carbon dopant concentration is in a range from about0.1% to about 5% (atomic percent). In some embodiments, silicon-basedmaterial can be a p-type doped silicon layer that is doped with boron.Other types of dopants for forming a p-type doped silicon layer may alsobe used, for example, gallium or indium. Those skilled in the art willrecognize that an embodiment illustrating a type of doping (e.g.,n-type) is generally applicable to use of an opposite type of doping(e.g., p-type). In alternative embodiments a p-type doped SiGe layer maybe epitaxially grown to form the S/D regions.

In some embodiments, silicon-based material can be formed by CVD, e.g.,low pressure CVD (LPCVD), atomic layer CVD (ALCVD), ultrahigh vacuum CVD(UHVCVD), PECVD, remote plasma CVD (RPCVD), any suitable CVD; molecularbeam epitaxy processes; any suitable epitaxial process; or anycombinations thereof.

In some embodiments, the etching process can use an etching gasincluding at least one of hydrogen chloride (HCl), chlorine (Cl₂), othersuitable etching gases, and/or any combinations thereof. The etchingprocess would remove the amorphous silicon-based material overnon-crystalline surface at a rate higher than the removal rate ofepitaxial silicon-based material. Therefore, only epitaxial film remainson the substrate surface after a CDDE cycle. The epitaxialdeposition/partial etch process is repeated a number of times until adesired thickness is reached.

Referring to FIGS. 3A and 3B, various exemplary structures resultingfrom fabrication operations for forming S/D contact structures havinglow contact resistance are shown.

In FIG. 3A, a top portion of doped drain region 110 _(D)′ is removed toform a drain region top surface 140, according to the cut 131illustrated in FIG. 1A. For ease of description, doped source region 110_(S)′ is not shown in FIG. 3A or 3B. Drain region top surface 140 may bea planar surface in parallel with fin top surface 114. Drain region topsurface 140 may additionally comprise sidewall portions that are not inparallel with fin top surface 114, which are not shown in FIG. 3A or 3B.For example, drain region top surface 140 may comprise portions that areperpendicular to fin top surface 114. The top portion of drain region110 _(D)′ may be removed using suitable processes includingphotolithography and etch processes. The photolithography process mayinclude forming a photoresist layer (resist) overlying the substrate(e.g., a finFET structure), exposing the resist to a pattern, performingpost-exposure bake processes, and developing the resist to form amasking element including the resist. The masking element may then beused to protect regions of the substrate while an etch process removes aleast a portion of doped S/D regions 110 _(D)′ and 110 _(S)′. The etchprocess may be RIE and/or other suitable processes. Numerous othermethods to etch doped S/D regions 110 _(D)′ and 110 _(S)′ may besuitable. As shown in FIG. 3A, drain region top surface 140 is above thefin top surface 114. In another embodiment, drain region top surface 140is below the fin top surface 114.

In various exemplary embodiments, an amorphous layer 142 is formed overdoped drain region 110 _(D)′ by a fabrication operation such as adeposition or a growth process. Amorphous layer 142 includes asemiconductor material or a semiconductor alloy material, and may be,specifically, an amorphous germanium layer, an amorphous silicon layer,an amorphous SiGe layer, or another amorphous semiconductor orsemiconductor alloy layer. Although the example of FIG. 3A depictsamorphous layer 142 being formed over doped drain region 110 _(D)′, inalternative embodiments, amorphous layer 142 is formed over otherregions of substrate 102. In some embodiments, semiconductor materialcan be formed by CVD, e.g., LPCVD, ALCVD, UHVCVD, PECVD, RPCVD, anysuitable CVD; any suitable deposition process; or any combinationsthereof. The thickness of amorphous layer 142 is controlled by thedeposition process.

An ion implantation is performed on amorphous layer 142, and may utilizeany suitable doping species. In-situ doping processes may also beincorporated into the deposition process of amorphous layer 142. Forexample, an n-type doping precursor, e.g., phosphine (PH₃) and/or othern-type doping precursors, can be used during the formation of n-type S/Dregions for an n-type FET. By using the in-situ doping process, thedopant concentration of silicon-based material can be desirablycontrolled and achieved. For example, amorphous layer 142 can be ann-type heavily-doped silicon layer that is doped with phosphorus (Si:P).In some embodiments, amorphous layer 142 can be an n-type doped siliconlayer that is doped with arsenic. Other types of dopants for formingn-type doped silicon layer may also be included. In some embodiments,the phosphorus dopant has a concentration in a range from about 5×10²⁰atoms/cm³ to at least about 7×10²¹ atoms/cm³. Amorphous layer 142 mayalso be a p-type heavily-doped silicon layer. For example, amorphouslayer 142 may be heavily doped with boron. Other types of dopants forforming p-type doped silicon layer may also be included, for example,gallium or indium.

FIG. 3B shows the structure of FIG. 3A after a crystallization processis performed on at least a portion of amorphous layer 142. In thisexemplary embodiment, the crystallization process converts amorphouslayer 142 to a crystalline layer by using the crystalline semiconductorstructure of epitaxial doped drain region 110 _(D)′ as a crystaltemplate.

An example method of crystallizing a layer of amorphous semiconductormaterial is a solid-phase epitaxial (SPE) regrowth process. The SPEregrowth process includes an annealing process and uses the crystallinesemiconductor structure of a seed layer as a crystal template tocrystallize an amorphous semiconductor layer. The SPE regrowth may beginat an interface between the seed layer and the amorphous layer andproceed through an entirety of a thickness of the amorphous layer. TheSPE regrowth may cause an entirety of the amorphous layer to become asingle-crystal semiconductor layer. In this exemplary embodiment, theannealing process may enable SPE regrowth to occur in amorphous layer142, starting from the interface of amorphous layer 142 and drain regiontop surface 140. The SPE regrowth may use the crystalline semiconductorstructure of epitaxial doped drain region 110 _(D)′ as a crystal seedlayer in crystallizing amorphous layer 142. As a result, the SPEregrowth crystallizes amorphous layer 142. In some embodiments,amorphous layer 142 is doped with impurities to facilitate or acceleratethe SPE regrowth. In other embodiments, the annealing andcrystallization processes may be performed multiple times to achievedesired results.

In some embodiments, the annealing process utilizes a temperature lowenough to prevent damage to the structure or to devices formed in thestructure. In one example, amorphous layer 142 includes an amorphousgermanium layer and epitaxial doped drain region 110 _(D)′ includes acrystalline germanium region, and an annealing temperature for thesolid-phase epitaxial regrowth is within a range of approximately 400 to600° C. In another example, amorphous layer 142 includes an amorphoussilicon layer and epitaxial doped drain region 110 _(D)′ includes acrystalline silicon region, and an annealing temperature for the SPEregrowth is under 600° C. In another example, amorphous layer 142includes an amorphous SiGe layer, and an annealing temperature for theSPE regrowth is in a range of 500-550° C. As a result of the SPEregrowth, amorphous layer 142 is converted to heavily-doped epitaxiallayer 142′.

Because the SPE regrowth allows heavily-doped epitaxial layer 142′ totake on the crystal orientation of a seed layer, a variety of differentcrystal orientations can be achieved for heavily-doped epitaxial layer142′. As described above, amorphous layer 142 may be comprised ofvarious semiconductor materials and semiconductor alloy materials. Forexample, the doped epitaxial source region 110 _(D)′ may be comprised ofcrystalline silicon and may be of various different crystalorientations, e.g., having a (100), (110), or (111) crystal orientation.The regrowth may cause amorphous layer 142 to take on the crystalorientation of the doped source region 110 _(D)′.

The SPE regrowth may also allow amorphous layer 142 and doped sourceregion 110 _(D)′ to be different materials. For example, a dopedepitaxial drain region 110 _(D)′ of SiGe may be used to crystallizeamorphous layer 142 comprising of either amorphous silicon or amorphousgermanium material. Specifically, when using doped epitaxial drainregion 110 _(D)′ of SiGe as a seed layer, crystallizing an amorphoussilicon layer or an amorphous germanium layer may cause stress or strainin the resulting crystalline silicon layer or crystalline germaniumlayer. For example, heavily-doped epitaxial layer 142′ of silicon formedusing SiGe as seed layer may be under tensile stress, where the tensilestress may increase electron mobility in the heavily-doped epitaxiallayer 142′. As another example, heavily-doped epitaxial layer 142′ ofgermanium formed using SiGe as seed layer may be under compressivestress, where the compressive stress may increase hole mobility inheavily-doped epitaxial layer 142′. Various other combinations of seedlayer and amorphous layer materials may be selected such that theresulting heavily-doped epitaxial layer 142′ may have a level of stressor strain.

The portions of amorphous layer 142 that are formed on areas of thesubstrate other than the doped S/D regions 110 _(S)′ and 110 _(D)′remain amorphous SPE regrowth, since they are not in contact with acrystalline seed layer. The remaining amorphous material is thenselectively removed through an etching process, for example, an RIEprocess, a chemical etching process (e.g., an HNO₃:HF dilute solution),and/or other suitable processes. Amorphous material etches much morerapidly than crystallized material, therefore sufficient etchselectivity can be obtained to selectively remove the remainingamorphous material. Depending on the device design and specific needs ofthe semiconductor structure, the thickness of heavily-doped epitaxiallayer 142′ may be in a range of 1-7 nm.

A contact layer 144 may then be formed upon heavily-doped epitaxiallayer 142′ of S/D regions 110 _(S)′ and 110 _(D)′. Contact layer 144 maybe formed by a self-aligned silicide (salicide) process. A salicideprocess involves deposition of, for example, a transition metal to forma thin layer by a suitable process such as CVD, application of heat toallow the transition metal to sinter with exposed material in the activeregions (source and drain), for example, heavily-doped epitaxial layer142′, to form a low-resistance transition metal silicide. Typicaltransition metal may include nickel, cobalt, tungsten, tantalum,titanium, platinum, erbium, palladium, or combinations thereof. Contactlayer 144 may include silicide materials, such as nickel silicide (NiSi,NiSi₂), nickel-platinum silicide (NiPtSi), nickel-platinum-germaniumsilicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbiumsilicide (YbSi), platinum silicide (PtSi), erbium silicide (ErSi),cobalt silicide (CoSi₂), titanium silicide (TiSi₂), tantalum silicide(TaSi₂), other suitable conductive materials, and/or combinationsthereof. Any remaining transition metal may be removed by chemicaletching, leaving silicide contacts only in the active regions. S/Dcontact plugs (not shown) may be further formed over the silicidematerial in subsequent process steps. In some embodiments, contact plugscomprise tungsten. In alternative embodiments, contact plugs compriseother metal or metal alloys such as aluminum, copper, titanium nitride(TiN), or the like. Contact plugs can be formed using appropriatedeposition and etching methods.

Heavily-doped epitaxial layer 142′ reduces the contact resistance atleast by enhancing the tunneling effect of charge carriers in S/Dregions. In accordance with this disclosure, a desired amount of contactresistance can be achieved by suitably adjusting parameters ofheavily-doped epitaxial layer 142′. In some embodiments, the contactresistance can be reduced at least by adjusting the doping level ofheavily-doped epitaxial layer 142′, for example, a higher doping levelwill result in a lower contact resistance. In some embodiments, thecontact resistance can be reduced by enlarging the contact area betweenheavily-doped epitaxial layer 142′ with the S/D regions of transistorregion 150. In some embodiments, the contact resistance can be reducedby increasing the thickness of heavily-doped epitaxial layer 142′.

Subsequent processing may further form various contacts/vias/lines andmultilayer interconnect features (e.g., metal layers and interlayerdielectrics) over substrate 102, configured to connect various featuresor structures. The additional features may provide electricalinterconnection to the device. For example, a multilayer interconnectionincludes vias and horizontal interconnects, such as metal lines. Thevarious interconnection features may be formed from various conductivematerials including, but not limited to, copper, and tungsten. In someembodiments, a damascene and/or dual damascene process is used to form aconductive multilayer interconnection structure.

One benefit of the heavily-doped contact layer in semiconductorstructures in accordance with this disclosure is that the contactresistance can be reduced compared to other contact structures. Anotherbenefit of the heavily-doped contact layer in accordance with thisdisclosure is that both n-type and p-type doping can be achieved withvery high doping levels in the contact layer and minimal diffusion intothe underlying material. The heavily-doped contact layer may also leavea minimal footprint in the semiconductor structure since the layerthickness can be precisely controlled by the SPE process and can bereduced to only a few nanometers. There may be no changes in layoutdesign rules since the heavily-doped contact is self-aligned to the S/Dregions. Further, layout design rules for incorporating a heavily-dopedcontact layer between S/D regions and the silicide contact layer inn-channel finFET S/D structures are the same as the layout design rulesfor p-channel finFET S/D structures.

Referring to FIGS. 4A and 4B, various exemplary structures resultingfrom fabrication operations for forming S/D contact structures havinglow contact resistance are shown.

In FIG. 4A, similar to the process described above in connection withFIG. 3A, a top portion of doped drain region 110 _(D)′ is removed toform a drain region top surface 140. For ease of description, dopedsource region 110 _(S)′ is not shown in FIG. 4A or 4B. Drain region topsurface 140 may be a planar surface in parallel with fin top surface114. Drain region top surface 140 may additionally comprise sidewallportions. For example, drain region top surface 140 may compriseportions that are perpendicular to fin top surface 114. The top portionof drain region 110 _(D)′ may be removed using suitable processesincluding photolithography and etch processes. However, as shown in FIG.4A, drain region top surface 140 is etched to a level that is below thefin top surface 114, and a fin sidewall portion 146 of fin 104 isexposed as a result of the etching process. Furthermore, a desiredheight difference between drain region top surface 140 and fin topsurface 114, as well as a desired amount of exposed fin sidewall portion146 can be achieved at least through adjusting etching parameters andconditions.

Similar to the process described above in connection with FIG. 3A, anamorphous layer 142 may be formed over doped drain region 110 _(D)′ by adeposition or growth process, and may further be doped by an ion implantprocess. Amorphous layer 142 may include a semiconductor material or asemiconductor alloy material, such as but not limited to, an amorphousgermanium layer, an amorphous silicon layer, an amorphous SiGe layer, oranother amorphous semiconductor or semiconductor alloy layer. FIG. 4Adepicts amorphous layer 142 disposed over drain region top surface 140.And since fin sidewall portions 146 are also exposed, amorphous layer142 is also deposited on fin sidewall portion 146. Similar to theprocess described with reference to FIG. 3A, amorphous layer 142 mayalso be formed over other regions of substrate 102.

In FIG. 4B, similar to the process described above with reference toFIG. 3B, annealing is used to crystallize amorphous layer 142 using thecrystalline semiconductor structure of the epitaxial doped drain region110 _(D)′ as a crystal template. If fin 104 comprises crystallinesemiconductor material, the portions of amorphous layer 142 that are incontact with fin sidewall portions 146 will also be crystallized usingthe crystalline semiconductor structure of fin sidewall portion 146 as acrystal template. The annealing may enable an SPE regrowth to occur inamorphous layer 142, starting from the interface of amorphous layer 142and drain region top surface 140, and also from the interface ofamorphous layer 142 and fin sidewall portion 146. The SPE regrowth usesthe crystalline semiconductor structure of epitaxial doped drain region110 _(D)′ and fin sidewall portion 146 as crystal orientation templatesin crystallizing amorphous layer 142. As a result, the SPE regrowthcrystallizes amorphous layer 142 in accordance with the crystalorientation of drain region 110 _(D)′ and fin sidewall portion 146. Insome embodiments, amorphous layer 142 is doped with impurities tofacilitate or accelerate the SPE regrowth. In other embodiments, theannealing and crystallization process may be performed multiple times toachieve desired results. In various embodiments, the annealing processutilizes a temperature low enough to prevent damage to the structure orto devices formed in the structure. As a result of the SPE regrowth,amorphous layer 142 is converted to heavily-doped epitaxial layer 142′.

As described above, the SPE regrowth provides heavily-doped epitaxiallayer 142′ to take on the crystal orientation of drain region 110 _(D)′and fin sidewall portion 146. The SPE regrowth allows amorphous layer142 and doped source region 110 _(D)′ to be different materials. Theportions of amorphous layer 142 that are formed on areas of thesubstrate other than doped S/D regions 110 _(S)′ and 110 _(D)′ remainamorphous during the SPE regrowth, since they are not in contact with acrystalline orientation template. The uncrystallized amorphous materialis then selectively removed through an etching process.

Contact layer 144 may then be formed upon heavily-doped epitaxial layer142′ of S/D regions 110 _(S)′ and 110 _(D)′. Contact layer 144 may beformed by a salicide process. For example, deposition of a transitionmetal to form a thin layer by a suitable process such as CVD,application of heat to allow the transition metal to sinter with exposedmaterial in the active regions (source and drain), for example,heavily-doped epitaxial layer 142′, to form a low-resistance transitionmetal silicide. Any remaining transition metal may be removed bychemical etching, leaving silicide contacts only in the active regions.S/D contact plugs (not shown) may be further formed over the silicidematerial in subsequent process steps. In some embodiments, contact plugscomprises tungsten. In alternative embodiments, contact plugs compriseother metal or metal alloys such as aluminum, copper, or the like.Contact plugs can be formed using appropriate deposition and etchingmethods.

One benefit of exposing fin sidewall portion 146 is that additionalcontact area between heavily-doped epitaxial layer 142′ and transistorregion 150 through fin sidewall portion 146 will decrease contactresistance. Another benefit of the heavily-doped contact layer inaccordance with this disclosure is that both n-type and p-type dopingcan be achieved with very high doping levels in the heavily-dopedepitaxial layer and minimal diffusion into the underlying material.Further, layout design rules for incorporating a heavily-doped contactlayer between S/D regions and the silicide contact layer in n-channelfinFET S/D structures are the same as the layout design rules forp-channel finFET S/D structures.

FIG. 5 is a flow diagram of an illustrative method 500 of forming S/Dstructures having lower contact resistance as compared to previouslyused S/D structures. Other fabrication steps may be performed betweenthe various steps of method 500, and are omitted merely for clarity.

Method 500 begins with a semiconductor substrate. For example, thesemiconductor substrate is a bulk Si wafer. Alternative embodiments mayuse other semiconductor materials. Method 500 includes operation 502,patterning a semiconductor substrate to form a fin. The fin is vertical,i.e., it is nominally perpendicular to the surface of the substrate, andthe fin may be rectangular or trapezoidal. In some embodiments the finmay have rounded corners where its top surface and sidewalls meet. Thefin may be formed using a variety of dry etch techniques such asreactive ion etching or inductively coupled plasma etching.

Method 500 continues with operation 504, forming a gate stack on thefin, the gate stack having a first sidewall and a second sidewall.Forming the gate stack includes forming a gate dielectric on the fin,and then forming a gate electrode over the gate dielectric. Examples ofgate dielectrics include, but are not limited to, one or more of silicondioxide, silicon nitride, and high-k dielectric materials. The gateelectrode may include a stack of various metal and metal alloy layers,or polysilicon.

Method 500 continues with operation 506, forming a first sidewall spaceradjacent to the first sidewall, and a second sidewall spacer adjacent tothe second sidewall. The first and second sidewall spacers are typicallyformed at the same time by an etch-back of a blanket layer. Inalternative embodiments the first and second sidewall spacers may beformed of two or more layers of material.

Method 500 continues with operation 508, performing LDD ion implantationon substrate 102 to dope LDD regions 113. LDD regions 113 are formed inthe fin structure between opposing spacers. An ion implantation isperformed to form LDD regions 113, and may utilize any suitable dopingspecies. After the dopant ions are implanted, a thermal anneal isperformed to drive in and to activate the dopants.

Method 500 continues with operation 510, etching exposed portions of thefin. These exposed portions of the fin are those portions that are notcovered by the gate stack or spacers. Because the gate stack and thespacers act as masking materials, they protect the fin underneath themfrom being etching. This etching may continue until the etched portionsof the fin are recessed below the neighboring isolation material. Thisetching process may also stop before the etched portions of the fin arerecessed below the neighboring isolation material. This exposed recessedinterface acts as a nucleation site for subsequent epitaxial growth ofmaterials.

Method 500 continues with operation 512, epitaxially growing material onthe recessed interface to form S/D regions. The epitaxially-grownmaterial may be a silicon-based material and may be formed by anepitaxial deposition/partial etch process. The process forms epitaxialS/D regions, 110 _(S)′ and 110 _(D)′, in recesses 127. Doping processesmay also be incorporated in-situ or after the deposition ofsilicon-based material. Doped epitaxial S/D regions are alsoself-aligned with the opening defined by opposing spacers 111.

Method 500 continues with operation 514, etching silicon-based materialin the S/D regions. A top portion of doped drain region 110 _(D)′ isremoved to form a drain region top surface 140. For ease of description,doped source region 110 _(S)′ is not shown in the figures. Drain regiontop surface 140 may be a planar surface in parallel with fin top surface114. Drain region top surface 140 can take the form of any suitableshape configuration, and may additionally comprise sidewall portionsthat are not in parallel with fin top surface 114. The top portion ofdrain region 110 _(D)′ is removed using suitable processes includingphotolithography and etch processes. Drain region top surface 140 may beabove the fin top surface 114, in accordance with some embodiments.Drain region top surface 140 may be below the fin top surface 114, inaccordance with some other embodiments.

Method 500 continues with operation 516, forming heavily-doped epitaxialcontact material in the S/D regions. First, an amorphous layer 142 maybe formed substantially over the doped S/D regions 110 _(S)′ and 110_(D)′ by a deposition or growth process. Amorphous layer 142 may includea semiconductor material or a semiconductor alloy material. Amorphouslayer 142 may also be formed substantially over other regions ofsubstrate 102. The thickness of amorphous layer 142 may be preciselycontrolled by the deposition process.

An ion implant process can be performed in-situ or after the depositionof amorphous layer 142, and may utilize any suitable doping species.Both n-type and p-type transistors can be fabricated using appropriatedoping procedures. The dopant concentration of amorphous silicon-basedmaterial can be adjusted to a higher level compared to a maximum dopantconcentration of crystalline silicon-based material, providing thebenefit of lowering the contact resistance.

A crystallization process is then used to crystallize the amorphouslayer 142 using the crystalline semiconductor structures of epitaxialdoped drain region 110 _(D)′ and/or fin sidewall portion 146 as crystaltemplates. An example method of crystallizing a layer of amorphoussemiconductor material is the SPE regrowth process. The annealingprocess may enable SPE regrowth to occur in the amorphous layer 142,starting from the interface of amorphous layer 142 and drain region topsurface 140, and/or the interface of amorphous layer 142 and finsidewall portion 146. As a result of the SPE regrowth, amorphous layer142 is converted to heavily-doped epitaxial layer 142′.

The SPE regrowth may also allow heavily-doped epitaxial layer 142′ totake on the crystal orientation of the seed layer and may thus allowheavily-doped epitaxial layer 142′ to have a variety of differentcrystal orientations. The SPE regrowth process may also allow amorphouslayer 142 and doped source region 110 _(D)′ to be different materials.

Uncrystallized amorphous material is selectively removed through anetching process. The thickness of heavily-doped epitaxial layer 142′ maybe in a range of 1-10 nm.

Method 500 continues with operation 518, forming a contact layer in theS/D regions for providing electrical connections. In this exemplaryembodiment, the contact layer is a low-resistance metal silicide.

Incorporating a heavily-doped contact layer between a S/D region and asilicide contact layer provides the benefit of reduced contactresistance compared to other contact structures. Another benefit of theheavily-doped contact layer in accordance with this disclosure is thatboth n-type and p-type doping can be achieved with very high dopinglevels in the contact layer and minimal diffusion into the underlyingmaterial. The thin heavily-doped contact layer may also leave a minimalfootprint in a semiconductor structure. No change in layout design rulesis needed since the heavily-doped contact is self-aligned to the S/Dregion. Further, layout design rules for incorporating a heavily-dopedcontact layer in n-channel finFET S/D structures are the same as thelayout design rules for p-channel finFET S/D structures.

In one embodiment, a method of forming semiconductor structure withreduced contact resistance includes forming a fin on a substrate andforming a gate structure on the fin, the gate structure having a firstsidewall and an opposing second sidewall. The method further includesforming a first sidewall spacer adjacent the first sidewall and forminga first source/drain (S/D) adjacent the first sidewall spacer. Adielectric layer is formed over the gate structure, the first sidewallspacer, and first S/D. A contact opening is formed through thedielectric layer such that a portion of the first S/D structure isexposed. A layer of doped amorphous material is formed over the gatestructure, the first sidewall spacer, and the exposed portion of thefirst S/D. The method further includes crystallizing a portion of thelayer of doped amorphous material to form a region of crystallizedmaterial.

In another embodiment, a semiconductor structure includes a fin on asubstrate and a gate structure on the fin. The gate structure isconfigured to have a first sidewall and an opposing second sidewall, afirst sidewall spacer adjacent the first sidewall, and a firstsource/drain (S/D) adjacent the first sidewall spacer. The semiconductorstructure further comprises a dielectric layer over the gate structure,the first sidewall spacer, and first S/D. A contact opening isconfigured to be formed through the dielectric layer such that a portionof the first S/D is exposed, and a layer of doped crystalline materialis configured to be on the exposed portion of the first S/D.

In a further embodiment, a structure comprises a fin over a substrate,the fin having a top surface and a pair of opposing side surfaces, and agate structure on the fin, the gate structure having a first sidewall.The structure further comprises a first sidewall spacer adjacent thefirst sidewall and a recess in the fin and adjacent the first sidewallspacer. The recess is configured to have a bottom surface and asidewall. The structure also comprises a first source/drain (S/D), aportion of which is disposed in the recess, and a layer of dopedcrystalline material on the first S/D and directly on a portion of therecess sidewall.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The foregoing disclosure outlines features of several embodiments sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art will appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art will also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A structure comprising; a fin over a substrate; agate structure, on the fin, having a sidewall; a sidewall spaceradjacent to the sidewall; a source/drain (S/D) region adjacent to thesidewall spacer; and a layer of doped crystalline material disposed onthe S/D region, wherein the layer of doped crystalline materialcomprises a higher doping concentration than a doping concentration ofthe S/D region.
 2. The structure of claim 1, wherein the layer of dopedcrystalline material is disposed in the fin.
 3. The structure of claim1, wherein the layer of doped crystalline material comprises a layer ofdoped crystalline semiconductor material.
 4. The structure of claim 3,wherein the layer of doped crystalline semiconductor material comprisesa layer of n-doped crystalline silicon with a doping concentration in arange of about 5×10²⁰ atoms/cm³ to about 7×10²¹ atoms/cm³.
 5. Thestructure of claim 3, wherein the layer of doped crystallinesemiconductor material comprises at least one of a layer of n-dopedcrystalline silicon and a layer of n-doped crystalline germanium.
 6. Thestructure of claim 3, wherein the layer of doped crystallinesemiconductor material comprises a layer of n-doped crystalline silicongermanium with a doping concentration in a range of about 5×10²⁰atoms/cm³ to about 7×10²¹ atoms/cm³.
 7. The structure of claim 1,further comprising a contact layer formed on the layer of dopedcrystalline material.
 8. The structure of claim 1, wherein a thicknessof the layer of doped crystalline material is in a range of about 1 nmto about 10 nm.
 9. The structure of claim 1, wherein the dopedcrystalline material and a material of the S/D region have a samecrystal orientation.
 10. The structure of claim 1, wherein the dopedcrystalline material and a material of the S/D region are formed of sameelements.
 11. The structure of claim 1, wherein the doped crystallinematerial is different from a material of the S/D region.
 12. Thestructure of claim 7, wherein the contact layer comprises a silicidelayer.
 13. A structure comprising; a fin, over a substrate, having a topsurface and a pair of opposing side surfaces; a gate structure, on thefin, having a sidewall; a sidewall spacer adjacent to the sidewall; asource/drain (S/D) region in the fin and adjacent to the sidewallspacer, a portion of the source/drain (S/D) region being disposed in thefin; and a layer of doped crystalline material on the S/D region andformed on the side surface of the fin, wherein the layer of dopedcrystalline material comprises a higher doping concentration than adoping concentration of the S/D region.
 14. The structure of claim 13,wherein the layer of doped crystalline material comprises a layer ofn-doped crystalline silicon with a doping concentration in a range ofabout 5×10²⁰ atoms/cm³ to about 7×10²¹ atoms/cm³.
 15. The structure ofclaim 13, wherein the layer of doped crystalline material comprises alayer of n-doped crystalline silicon germanium with a dopingconcentration in a range of about 5×10²⁰ atoms/cm³ to about 7×10²¹atoms/cm³.
 16. The structure of claim 13, wherein a thickness of thelayer of doped crystalline material is in a range of about 1 nm to about10 nm.
 17. A semiconductor structure comprising: a fin on a substrate; afirst gate structure, over the fin, with a first sidewall and a firstspacer formed thereon; a second gate structure, over the fin, with asecond sidewall and a second spacer formed thereon, the second spaceropposing the first spacer; a crystalline source/drain (S/D) regionformed in the fin and between the first spacer and the second spacer;and a crystalline layer formed on the crystalline S/D region, whereinthe crystalline layer comprises a higher doping concentration than adoping concentration of the crystalline S/D region.
 18. Thesemiconductor structure of claim 17, wherein a thickness of thecrystalline layer is in a range of about 1 nm to about 10 nm.
 19. Thesemiconductor structure of claim 17, wherein the crystalline layercomprises an n-doped crystalline silicon layer with a dopingconcentration in a range of about 5×10²⁰ atoms/cm³ to about 7×10²¹atoms/cm³.
 20. The semiconductor structure of claim 17, wherein thecrystalline layer comprises an n-doped crystalline silicon germaniumlayer with a doping concentration in a range of about 5×10²⁰ atoms/cm³to about 7×10²¹ atoms/cm³.